The invention relates to an appliance for the oxidation of components in organic samples, in particular for the oxidation of components emerging from a gas chromatograph and for preparation for an analysis of isotope ratios by mass spectrometry, having a chamber with a gas inlet and a gas outlet, having an oxidizer inside the chamber and, in particular, having a heating appliance for the chamber, characterized in that the oxidizer is nickel oxide. The invention furthermore relates to a process for isotope ratio analysis.
One of the many possible uses of investigations by mass spectrometry is the analysis of isotope ratios. Thus, the analysis of carbon and nitrogen isotopes in components in organic samples is of interest. It is known for the investigation of gaseous components to arrange a gas chromatograph upstream of the mass spectrometer and to separate the individual components in time. It is furthermore known to oxidize the components before carrying out the analysis by mass spectrometry. In this case, for example the carbon in the organic sample is converted into CO.sub.2 and the hydrogen into H.sub.2 O. CO.sub.3 results as the product of oxidation of various components. This means that the analysis can be carried out on the basis of a single compound (CO.sub.2). Furthermore, it is relatively simple to manipulate CO.sub.2 because it can be introduced at room temperature in the form of a gas without heated lines into the ion source of the mass spectrometer. The oxidation of the organic components takes place in a combustion oven (combusion interface). In this the organic components are heated and oxidized with the aid of a special oxidizer. This process is known in the literature as IRM-GCMS (isotope ratio monitoring gas chromatography/mass spectrometry) or GC-IRMS (gas chromatography isotope ratio mass spectrometry). The oxidizer regarded as particularly advantageous to date has been copper oxide (CuO). Corresponding analytical processes and relevant apparatus are described in Analytical Chemistry, Vol. 50, No. 11, September 1978, pages 1465-1473 and Org. Geochem. Vol. 16, Nos. 4-6, 1990, pages 1115-1128. The oxidizer is, as a rule, provided with a supplementary platinum catalyst.
The use of copper oxide as oxidizer has various disadvantages. A high temperature is necessary for the oxidation of the organic components, for example a temperature near 1000.degree. C. for methane. At this temperature the copper oxide is decomposed to Cu and O.sub.2. This means that the oxidation of the organic components is incomplete. To avoid this effect it is necessary for the copper to be continually reoxidized. Appropriate additional appliances must be provided. In addition, at such high temperatures there is even coagulation of the copper, which thus substantially loses its oxidizing ability. Despite these disadvantages, to date copper oxide (CuO) has been regarded as the most suitable oxidizer.